1. Field of the Invention
The invention relates to a method of shaping a bell end on a pipe of thermoplastic material, the opening of the bell end having a diameter which is smaller than the maximum diameter of the inner surface of the bell end and substantially equal to the outer diameter of the pipe, by means of a cylindrical shaping mandril which is to be axially inserted into the pre-heated pipe end.
2. Description of the Prior Art
In a method of this kind known from U.S. Pat. No. 3,853,450 issued Dec. 10, 1974 to the instant applicant, the shaping mandril comprises a base part a shaping ring of deformable material slidable on the base part, and a sleeve part slidable with respect to the base part, said base and sleeve parts being profiled such that in the operative position of the shaping mandril, the shaping ring is locked against axial and radial deformation in an annular chamber formed between the base part and the sleeve part, the outer surface of the shaping mandril having cylindrical portions of step-like varying diameters with conical transition portions. The shaping ring comprises at least one cylindrical portion having a diameter which is larger than the other diameter of the pipe, the base part and the sleeve part having a cylindrical portion whose diameter is substantially equal to the outer diameter of the pipe, the sleeve part having a conical front portion adjoining the cylindrical portion thereof and which is forwards and radially inwards inclined, the smallest diameter thereof being at the most equal to the inner diameter of the pipe.
The bell end is then formed by relative axial movement between the shaping mandril in said operative position and the pipe, resulting in a step-wise expansion of the pre-heated pipe material. By means of an external tool, consisting for example of a pressure ring which can be coaxially secured with respect to the shaping mandril and a split, initially open shaping sleeve, the pressure ring abuts against the shaping sleeve in a given relative position of shaping mandril and pipe, the shaping sleeve being subsequently clamped about the pipe end. In order to release the shaping mandril, first the clamping between the two metal mandril parts is removed; subsequently the base part of the shaping mandril, and after that the shaping ring and the sleeve part, are axially removed from the formed bell end.
In the described known method, after the interior of the bell end has been made accessible by removal of the base part of the shaping mandril therefrom, the deformable shaping ring is first removed, using a suitable tool, from the bell end which has meanwhile cooled, and is subsequently slid by hand onto the base part again. After that, the sleeve part of the shaping mandril is removed from the bell end by means of a hook-like tool which can engage behind the sleeve part inside the bell end and which also serves, in the active position of the shaping mandril, for keeping the metal mandril parts clamped together, after which the shaping material can be brought into the active position again by means of said tool.